Multiple Machine Operation—Interference Is The Enemy

Interference is the term I use to describe when one machine sits idle while waiting for the operator to do something on another machine. For example, if both machines finish their cycles at precisely the same time, and if neither machine has an automatic loading device, one machine will sit idle waiting for the (one) operator to load the other machine.

Columns
From:
4/1/2008
Modern Machine Shop,
Mike
Lynch

Interference is the term I use to describe when one machine sits idle while waiting for the operator to do something on another machine. For example, if both machines finish their cycles at precisely the same time, and if neither machine has an automatic loading device, one machine will sit idle waiting for the (one) operator to load the other machine. It is experiencing interference during this time.

There are many less than ideal, yet still financially feasible, applications for having one operator run two machines. When the amount of interference is kept to a minimum—usually by design—the cost for having one operator run two machines will still be less than the cost of having a separate operator run each machine.

Do note that any interference will have an adverse affect on the total output potential of the two machines, and it will increase the amount of time it will take to complete a given job. This must be considered when making personnel-utilization decisions, especially when there is urgency to getting jobs done quickly, or maximizing output.

In poorly planned applications, interference can easily cause it to cost more to have one operator run two machines than to have a separate operator run each machine. This occurs more often than you might think.

Remember, the maximum cost gain from having one operator run two machines is directly related to the relationship between operator cost and machine cost. Combine expensive machines with a low-paid operator, and there isn’t much of a cost benefit.

In the worst situations, expensive machines sit idle for such long periods of time that it doesn’t make sense to have the second machine involved. One operator running one machine can keep up with—or even out-perform—one operator running two machines.

From a purely cost-related standpoint, there will always be a cutoff point when it just doesn’t make sense to have one operator run two machines. For these discussions, the cutoff point is when it costs just as much to have one operator run two machines as it does to have a separate operator run each machine. Given the fact that productivity also suffers, you may feel that even this situation is wasteful.

The only “benefit” I can think of to having one operator run two machines when you pass the cutoff point, is the reduction in required operators. Maybe your company can’t find and hire enough experienced people for the wages you’re able to pay. When you pass the cutoff point though, you’re losing money and wasting production time.

Remember, the only cost benefit to having one operator run two machines is that you save the cost of one operator. The more costly the machine (compared to the operator), the more quickly the interference will cause the cutoff point to be reached.

How Quickly Interference Can Add Up

Many companies expect a lot from their CNC operators. They expect operators to make setups and load parts. Once a part is completed, they expect operators to deburr it, inspect it and report their measurements to an SPC system. They expect operators to make offset adjustments to keep parts on size as well as to replace dull tools and maintain machines by keeping them clean.

Now, throw in the task of running a second machine—this effectively doubles what you expect the operators to do. Without forethought, planning, engineering and possibly the purchase of certain accessories, there’s going to be a lot of interference—and it is almost certain that the cutoff point will be exceeded.

Interference is the enemy. If you decide to use one operator to run two machines without considering it, the results will almost certainly be costly. The more you expect of your CNC operators, the more work you’ll have to do in order to engineer a feasible application for having one operator run two machines. This can be a very difficult and complex challenge.

If you expect your operators to make setups, for example, the machine being set up will experience interference whenever the operator must load parts, deburr parts, check parts and make adjustments to the machine that is in production. Also, the machine in production will experience interference whenever a cycle is completed. At the very least, the operator must break away from his or her current setup task and walk over to the machine to be loaded (more likely, the operator will work until he or she comes to a convenient stopping point before walking over to work on the machine in production). If the two machines happen to complete a production run at the same time, of course, one machine will sit idle for the entire length of time it takes to set up the other.

How can interference be eliminated or minimized? This will be your challenge when faced with marginal applications. With ingenuity and resources (money), you will be able to come up with methods that address each interference problem—but don’t wait until after your operator utilization decision is made to do so. Be sure to have workable solutions to all interference questions in place before you make the decision to have one operator run two machines.

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